from my understanding, you select the original bios and make sure your hard drive is locked and the chip is off and it's safe to go on live. i haven't tried this with my Xenium, but i just read it on xbox-scene 5 minutes ago.

just to fill in on the xenium... yeah, you can either press the button you assigned the original bios to, or wait for the xenium os to start. The original and all four banks are bootable and configurable from here.